1. Field of the Invention
The present invention relates to a multilayered ceramic substrate for mounting thereon and interconnecting LSI and chip components, and a method of manufacturing the same.
2. Description of the Prior Art
Conductor materials used for multilayered substrate include noble metals such as Au, Pt and Pd, and base metals such as W, Mo and Cu. On this metal material, an organic binder and a solvent are added to form into a paste, which is screen-printed on a dielectric substrate made of alumina or the like, and fired, thereby forming a conductor pattern. To manufacture a multilayered substrate, other methods are also known, such as a multilayering method by using, aside from these conductor pastes, ceramics and glass powder as insulating materials which are diffused in a solvent containing a dissolved organic binder, and a multilayering method by laminating formed patterns of conductor paste on a green sheet made of such insulating powder, organic binder and others. As for the metal conductor materials used in them, Au and Ag/Pd can be sintered in air, but they are noble metals and are expensive. On the other hand, W, Mo and Cu are base metals and inexpensive, but they must be sintered in a reducing atmosphere or inert atmosphere. Besides, W and Mo require a high sintering temperature of 1500.degree. C. or more. From the viewpoint of reliability, Au has a problem in solder leaching, and Ag/Pd is high in migration and conductor resistance. Accordingly, Cu is being noticed lately because it is low in conductor resistance, small in migration, and favorable in solderability.
To use such Cu, however, it is necessary as mentioned above to sinter in an inert atmosphere such as nitrogen. In nitrogen atmosphere, it is difficult to decompose and remove the organic binder in the paste. This is because the oxygen partial pressure in the nitrogen atmosphere is low, and decomposition of the binder does not occur, and it is left over in a form of carbon, which adversely affects the metallizing performance (sheet resistance, solderability, adhesion strength). To the contrary, where the oxygen partial pressure in the firing atmosphere during the firing step is high, the copper electrode is oxidized and the soldering performance becomes worse. In sintering, accordingly, it is required to control the oxygen slightly in nitrogen atmosphere. The same problems occur in the laminate using a green sheet such as multilayered ceramic capacitor.
That is, it is difficult to burn out the organic binder contained in the green sheet such as dielectric, and unless burnt out completely, the binder is left over in a carbonized state, which may induce blister between layers or worsen the matching of electrode and dielectric. A method to solve the contradictory problems of binder burn-out and Cu metallizing was proposed. This is a method of using copper oxide as a starting material of electrode, and according to this method, heat treatment is preliminarily conducted in the air in order to burn out the organic components contained in the paste, and then the copper oxide is reduced to obtain a metal copper, and then sintering is performed in nitrogen atmosphere. This method is ideal for obtaining a laminate of Cu electrode because of the complete removal of the organic components in the first place to be followed by reducing and sintering steps. This multilayering method by copper oxide is disclosed in U.S. Pat. No. 4,714,570, and the copper oxide paste is mentioned in U.S. Pat. No. 4,906,405.
In such constitution, on the other hand, the following problems to be solved are found out. That is, in such multilayered ceramic substrate, it is difficult to apply wire bonding directly on the top layer Cu electrode. The wire bonding on the Cu electrode involves many reliability problems at the present, which is because the Cu multilayered ceramic substrate is not used widely.
It has been therefore attempted to apply Au plating on the top layer Cu electrode, but the adhesion strength of Cu electrode is extremely lowered due to the effect of plating solution. On the other hand, when forming an Au electrode by other method than plating, since the inside of the multilayered substrate and the top layer printed pattern are both Cu electrodes, and in order to print and sinter the thick film Au paste, it is necessary to sinter in nitrogen in order to prevent oxidation of Cu. It, in turn, leads to problems in the solderability and adhesion strength of Au electrode. Besides, connection of Cu and Au cannot be treated at so high temperature because the junction is lowered in melting point due to the reaction of Au and Cu.
In the existing method, moreover, it is general to sinter the internal electrode and substrate material simultaneously, and then apply the top layer Cu printed pattern. In this case, therefore, the sintering shrinkage of the substrate is not uniform, but is variable depending on the lot fluctuations of the substrate materials or sintering condition, and hence it is necessary to use a land for connection, considering the allowance for fluctuations of the shrinkage rate, for connection of the top layer pattern and the inner layer pattern.
As a result, a wider land than necessary is present on the top layer pattern, which is an obstacle for higher density mounting.